Various mechanisms have been suggested for the central action. In 1992, Vane and Feirrera have considered the involvement of cyclooxygenase plants, but their conclusions seem to have been invalidated by more recent work showing that, compared to NSAIDs, acetaminophen is a weak inhibitor of these enzymes. Another hypothesized mechanism of action of acetaminophen involves central serotonergic system have demonstrated a reduction of the effect of paracetamol after lesion of descending serotonergic pathways. The inhibition of serotonin synthesis by p-chlorophenylalanine reduced the antinociceptive effect of paracetamol in the hot plate test and formalin test. In addition, paracetamol increases serotonin levels in the cortex and the deck and reduces the number of cortical 5-HT2 receptors, without changing the number of 5-HT1A receptors.